1. Field of the Invention
The present invention generally relates to high resolution, broad band x-ray microcalorimeters and specifically to the electronic readout for these x-ray spectrometers.
2. Description of Related Art
When operated below 300 mK, cryogenic microcalorimeters offer nearly 100% efficiency between 100 eV and 10 keV and an energy resolution of a few electron volts. Currently available x-ray detectors cannot match such capabilities simultaneously. As a consequence, microcalorimeters are being used to improve the spectroscopy of astrophysical and laboratory plasmas as well as enhancing the sensitivity of present-day x-ray fluorescence methods for trace element determination in biological specimens, geological and environmental waste samples. This includes microanalysis using electron excitation in scanning electron microscopes, x-ray fluorescence using synchrotron radiation and proton excitation.
In a microcalorimeter, x-ray photons are absorbed and thermalized in a detector which is weakly coupled thermally to a cold bath. The rise in the detector's temperature as a result of the x-ray absorption is measured with a thermal sensor, producing an electrical signal that is proportional to the x-ray energy. For operation at temperatures below 4K these thermal sensors, or thermistors, take advantage of the strong temperature dependence of resistance in doped semiconductor crystals such as silicon or germanium. The electrical resistance of the sensor is determined by hopping conduction of free carriers, a process that is characteristic of doped germanium at cryogenic temperatures. See, U.S. Pat. No. 5,777,336.
The semiconductor thermistor used in such a microcalorimeter is impedance-matched to a JFET negative voltage feedback circuit. See, Silver et al. SPIE, Vol. 1159 at 423 (1989). An energy resolution of 5.9 eV at 1.5 keV and 7 eV at 6 keV has been achieved with neutron transmutation-doped (NTD) germanium-based thermistor technology. The thermalization time of the NTD-based detectors is about 10-20 .mu.s and the thermal recovery time, .ltoreq.500 .mu.s.
Up to now, a microcalorimeter has required a matching JFET preamplifier to operate successfully. There are many space-based and industrial applications that could benefit from operating multi-element arrays of these microcalorimeters. The corresponding number of preamplifiers and data processors could seriously drain available resources if the array consists of 50 or more microcalorimeters and their associated preamplifiers and data processors. Multiplexing a single JFET preamplifier among several microcalorimeters would be extremely advantageous in such cases.